The role of leukocyte activation and leukocyte-endothelial cell interactions remains poorly understood and characterized in vasculitic pathologies. This laboratory has recently reported that ICAM-1 deficient MRL/MpJ-Fas/lpr mice demonstrate reduced vasculitic lesion formation and vessel necrosis. This project will investigate the hypothesis that ICAM-1/Mac-1 interactions between leukocytes and arterial endothelial cells mediate leukocyte activation, arterial endothelial cell damage, and subsequent arterial vessel necrosis observed in vasculitic lesions. The hypothesis will be examined using both Mac-1 and ICAM-1 mutant mice that have been backcrossed onto the MRL/MpJ-Fas/lpr background, a strain that spontaneously develops vasculitis. The first specific aim is to compare vasculitic lesion formation between Mac-1 mutant and Mac-1 wild type MRL/MpJ-Fas/lpr mice. The role of Mac-1 expression will be determined during various stages of vasculitis development using pathological grading studies. The second specific aim will determine the role of Mac-1/ICAM-1 interactions in leukocyte activation and endothelial cell damage in vitro. Mesenteric arterial endothelial cell cultures will be used to measure neutrophil adhesion of Mac-1- mutant and wild type neutrophils to both ICAM-1 mutant and wild type MRL/MpJ-Fas measure neutrophil adhesion of Mac-1 mutant and wild neutrophils to both ICAM-1 mutant and wild type MRL/MpJ-Fas lpr endothelial cells. Leukocyte activation and endothelial cell cytotoxicity will also be determined using Mac-1 mutant and wild type neutrophils with this in vitro model. The combined in vivo and in vitro approach of this project will significantly advance our understanding of the role of leukocyte adhesion and leukocyte mediated vessel necrosis in vasculitis.